Centrifugal compressor

ABSTRACT

A centrifugal compressor includes: a compressor impeller; a main flow passage in which the impeller is arranged, the main flow passage extending in a rotational axis direction of the impeller; a sub flow passage including an upstream communication passage communicating with the main flow passage and a downstream communication passage communicating with the main flow passage on the impeller side with respect to the upstream communication passage; N first partitions fixed to the sub flow passage and arranged while spaced apart from each other in a rotation direction of the impeller; and M second partitions (where N−4≤M≤N+4) fixed on the upstream communication passage side with respect to the first partitions in the sub flow passage, the M second partitions arranged while spaced apart from each other in the rotation direction.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of International Application No. PCT/JP2018/015851, filed on Apr. 17, 2018, which claims priority to Japanese Patent Application No. 2017-086557, filed on Apr. 25, 2017, the entire contents of which are incorporated by reference herein.

BACKGROUND ART Technical Field

The present disclosure relates to a centrifugal compressor in which a sub flow passage communicating with a main flow passage is formed.

Related Art

In some of centrifugal compressors, a sub flow passage communicating with a main flow passage is formed. A compressor impeller is arranged in the main flow passage. The main flow passage and the sub flow passage communicate with each other at an upstream communication passage and a downstream communication passage. In an area where the flow rate is small, the high-pressure air compressed by the compressor impeller flows backward through the downstream communication passage and the sub flow passage. The air flowing backward circulates from the upstream communication passage to the main flow passage. In this manner, an apparent flow rate increases, and thus the operation area on the small flow rate side expands.

In a centrifugal compressor described in Patent Literature 1, fixed vanes and movable vanes are provided in a sub flow passage. The movable vanes are arranged on the side farther from the impeller than the fixed vanes are. The sub flow passage is opened and closed by the movable vanes.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Patent No. 4798491

SUMMARY Technical Problem

Meanwhile, there are cases where first partitions are arranged like the above-mentioned fixed vanes are and second partitions are arranged while fixed instead of the movable vanes. The multiple first partitions and second partitions are spaced apart in the rotation direction of an impeller. The air flowing in from a downstream communication passage into a sub flow passage has a swirling speed component. The swirling speed component is suppressed by the first partition and the second partition, which expands the operation area. However, in a case where the number of second partitions is not appropriate relative to the number of first partitions, disadvantageously, the pressure loss increases.

An object of the present disclosure is to provide a centrifugal compressor capable of reducing pressure loss.

Solution to Problem

In order to solve the above disadvantages, a centrifugal compressor according to an aspect of the present disclosure includes: an impeller; a main flow passage in which the impeller is arranged, the main flow passage extending in a rotational axis direction of the impeller; a sub flow passage including an upstream communication passage communicating with the main flow passage and a downstream communication passage communicating with the main flow passage on the impeller side with respect to the upstream communication passage; N first partitions fixed to the sub flow passage and arranged while spaced apart from each other in a rotation direction of the impeller; and M second partitions (where N−4≤M≤N+4) fixed on the upstream communication passage side with respect to the first partitions in the sub flow passage, the M second partitions arranged while spaced apart from each other in the rotation direction.

A thickness of the first partitions in the rotation direction may be less than or equal to five times a thickness of the second partitions in the rotation direction.

The downstream communication passage may open between the multiple first partitions.

An inner wall surface of the upstream communication passage on a side spaced apart from the impeller may be inclined in a direction closer to the impeller as the inner wall surface is closer to the main flow passage.

The second partitions may extend in a radial direction of the impeller.

Effects of Disclosure

According to the present disclosure, it is possible to reduce pressure loss.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view of a turbocharger.

FIG. 2 is an extracted diagram of a broken line part in FIG. 1.

FIG. 3 is a graph for explaining the relationship between the flow rate of a main flow passage and the flow rate of a sub flow passage.

FIG. 4A is a cross-sectional view taken along line IVa-IVa in FIG. 2. FIG. 4B is a cross-sectional view taken along line IVb-IVb in FIG. 2.

FIG. 5 is a table illustrating exemplary measurement results of the compression efficiency depending on the number of ribs and the number of fins.

FIG. 6 is a first graph based on the measurement results illustrated in FIG. 5.

FIG. 7 is a second graph based on the measurement results illustrated in FIG. 5.

DESCRIPTION OF EMBODIMENTS

An embodiment of the present disclosure will be described in detail below with reference to the accompanying drawings. Dimensions, materials, other specific numerical values, and the like illustrated in embodiments are merely examples for facilitating understanding, and the present disclosure is not limited thereby unless otherwise specified. Note that, in the present specification and the drawings, components having substantially the same function and structure are denoted by the same symbol, and redundant explanations are omitted. Components not directly related to the present disclosure are not illustrated.

FIG. 1 is a schematic cross-sectional view of a turbocharger C. Descriptions are given assuming that a direction of an arrow L illustrated in FIG. 1 is the left side of the turbocharger C. Descriptions are given assuming that a direction of an arrow R illustrated in FIG. 1 is the right side of the turbocharger C. Of the turbocharger C, a compressor impeller 9 (impeller) side which will be described later functions as a centrifugal compressor. Hereinafter, the turbocharger C is described as an example of a centrifugal compressor. However, the centrifugal compressor is not limited to the turbocharger C. The centrifugal compressor may be incorporated in a device other than the turbocharger C or may be a separate device.

As illustrated in FIG. 1, the turbocharger C includes a turbocharger main body 1. The turbocharger main body 1 includes a bearing housing 2. A turbine housing 4 is connected to the left side of the bearing housing 2 by a fastening bolt 3. A compressor housing 100 is connected to the right side of the bearing housing 2 by a fastening bolt 5.

A bearing hole 2 a is formed in the bearing housing 2. The bearing hole 2 a penetrates through the turbocharger C in the left-right direction. A bearing 6 is provided in the bearing hole 2 a. In FIG. 1, a full-floating bearing is illustrated as an example of the bearing 6. However, the bearing 6 may be another radial bearing such as a semi-floating bearing or a rolling bearing. A shaft 7 is pivotally supported by the bearing 6 in a freely rotatable manner. At the left end of the shaft 7, a turbine impeller 8 is provided. The turbine impeller 8 is accommodated in the turbine housing 4 in a freely rotatable manner. Furthermore, a compressor impeller 9 is provided at the right end of the shaft 7. The compressor impeller 9 is accommodated in the compressor housing 100 in a freely rotatable manner.

A housing hole 110 is formed in the compressor housing 100. The housing hole 110 opens to the right side of the turbocharger C. A mounting member 200 is arranged in the housing hole 110. A main flow passage 10 is formed by the compressor housing 100 and the mounting member 200. The main flow passage 10 opens to the right side of the turbocharger C. The main flow passage 10 extends in the rotational axis direction of the compressor impeller 9 (hereinafter simply referred to as the rotational axis direction). The main flow passage 10 is connected to an air cleaner (not illustrated). The compressor impeller 9 is arranged in the main flow passage 10.

In a state in which the bearing housing 2 and the compressor housing 100 are connected by the fastening bolt 5, a diffuser flow passage 11 is formed. The diffuser flow passage 11 is formed by opposing surfaces of the bearing housing 2 and the compressor housing 100. The diffuser flow passage 11 pressurizes the air. The diffuser flow passage 11 is annularly formed outward from an inner side in the radial direction of the shaft 7. The diffuser flow passage 11 communicates with the main flow passage 10 at the inner side in the radial direction.

Furthermore, the compressor housing 100 is provided with a compressor scroll flow passage 12. The compressor scroll flow passage 12 is annular. The compressor scroll flow passage 12 is positioned, for example, on an outer side of the diffuser flow passage 11 in the radial direction of the shaft 7. The compressor scroll flow passage 12 communicates with an intake port of an engine (not illustrated). The compressor scroll flow passage 12 also communicates with the diffuser flow passage 11. When the compressor impeller 9 rotates, the air is sucked into the compressor housing 100 from the main flow passage 10. The sucked air is accelerated and pressurized in the process of flowing through blades of the compressor impeller 9. The accelerated and pressurized air is further pressurized by the diffuser flow passage 11 and the compressor scroll flow passage 12. The pressurized air is guided to the intake port of the engine.

A discharge port 13 is formed in the turbine housing 4. The discharge port 13 opens to the left side of the turbocharger C. The discharge port 13 is connected to an exhaust gas purification device (not illustrated). The turbine housing 4 is provided with a flow passage 14 and a turbine scroll flow passage 15. The turbine scroll flow passage 15 is annular. The turbine scroll flow passage 15 is positioned, for example, on an outer side of the flow passage 14 in the radial direction of the turbine impeller 8. The turbine scroll flow passage 15 communicates with a gas inlet port (not illustrated). Exhaust gas discharged from an exhaust manifold of the engine (not illustrated) is guided to the gas inlet port. The gas inlet port also communicates with the flow passage 14 described above. The exhaust gas guided from the gas inlet port to the turbine scroll flow passage 15 is guided to the discharge port 13 via the flow passage 14 and between blades of the turbine impeller 8. The exhaust gas guided to the discharge port 13 rotates the turbine impeller 8 in the process of flowing therethrough.

The turning force of the turbine impeller 8 is then transmitted to the compressor impeller 9 via the shaft 7. As described above, the turning force of the compressor impeller 9 causes the air to be pressurized and guided to the intake port of the engine.

FIG. 2 is a diagram of a broken line part extracted from FIG. 1. As illustrated in FIG. 2, a partition wall 120 is formed in the housing hole 110. The partition wall 120 is annular. The partition wall 120 extends in the rotational axis direction. The partition wall 120 is spaced apart radially inward from the inner circumferential surface of the housing hole 110. The inner circumferential surface of the housing hole 110 and the outer circumferential surface of the partition wall 120 are parallel to the rotational axis direction. However, the inner circumferential surface of the housing hole 110 and the outer circumferential surface of the partition wall 120 may be inclined with respect to the rotational axis direction, or may not be parallel to each other.

A protrusion 130 is formed on a bottom surface 111 of the housing hole 110. The protrusion 130 is annular. The protrusion 130 extends in the rotational axis direction. The protrusion 130 is spaced apart radially inward from the inner circumferential surface of the housing hole 110. The outer circumferential surface of the protrusion 130 is parallel to the rotational axis direction. However, the outer circumferential surface of the protrusion 130 may be inclined with respect to the rotational axis direction.

The outer circumferential surface of the partition wall 120 and the outer circumferential surface of the protrusion 130 are flush. Note that the outer diameter of the partition wall 120 may be larger or smaller than the outer diameter of the protrusion 130. An end surface 121 of the partition wall 120 on the left side (protrusion 130 side) in FIG. 2 and an end surface 131 of the protrusion 130 on the right side (partition wall 120 side) in FIG. 2 are spaced apart in the rotational axis direction. A slit (downstream communication passage 310 which will be described later) is formed between the end surface 121 of the partition wall 120 and the end surface 131 of the protrusion 130.

Ribs 140 (first partitions) are formed in the housing hole 110. Multiple ribs 140 are spaced apart from each other in the circumferential direction of the partition wall 120 (the rotation direction of the compressor impeller 9). In FIG. 2, a rib 140 is indicated by cross hatching to facilitate understanding. The ribs 140 are integrally molded with the bottom surface 111 of the housing hole 110. The ribs 140 protrude from the bottom surface 111 to the right side (fin side which will be described later) in FIG. 2. The ribs 140 are integrally molded also with the inner circumferential surface of the housing hole 110 and the outer circumferential surface of the partition wall 120. That is, the partition wall 120 is integrally molded with the compressor housing 100. The partition wall 120 is held by the ribs 140 with a gap maintained from the housing hole 110. However, the partition wall 120 may be formed separately from the compressor housing 100 and attached to the compressor housing 100.

A partition wall hole 122 is formed in the partition wall 120. The partition wall hole 122 penetrates through the partition wall 120 in the rotational axis direction. In the partition wall hole 122, a large diameter portion 122 a, a reduced diameter portion 122 b, and a small diameter portion 122 c are formed. The large diameter portion 122 a opens at an end surface 123 of the partition wall 120 on the right side (opposite side to the protrusion 130) in FIG. 2. The reduced diameter portion 122 b is continuous with the left side (protrusion 130 side) of the large diameter portion 122 a in FIG. 2. The inner diameter of the reduced diameter portion 122 b decreases toward the left side (protrusion 130 side) in FIG. 2. The inner diameter of the small diameter portion 122 c is smaller than the inner diameter of the large diameter portion 122 a. The small diameter portion 122 c is continuous with the left side (protrusion 130 side) of the reduced diameter portion 122 b in FIG. 2. In this example, the case where the large diameter portion 122 a, the reduced diameter portion 122 b, and the small diameter portion 122 c are formed has been described. However, as long as the partition wall hole 122 is formed, the shape thereof does not matter.

A protrusion hole 132 is formed in the compressor housing 100. The protrusion hole 132 penetrates through the protrusion 130 in the rotational axis direction. The protrusion hole 132 faces the partition wall hole 122. A part of the compressor impeller 9 is arranged in the protrusion hole 132 and the partition wall hole 122. The inner circumferential surface of the protrusion hole 132 matches the outer shape of the compressor impeller 9. The inner diameter of the protrusion hole 132 becomes smaller toward the right side (partition wall hole 122 side) in FIG. 2. The partition wall hole 122 and the protrusion hole 132 form a part of the main flow passage 10 described above.

In the compressor housing 100, the housing hole 110 opens at an end surface 100 a on the right side (opposite side to the turbine impeller 8) in FIG. 2. As described above, the mounting member 200 is arranged in the housing hole 110. The main body portion 210 of the mounting member 200 is, for example, annular. The main body portion 210 is not limited to an annular shape, and for example, a part of the main body portion 210 in the circumferential direction may be cut away.

The main body portion 210 is press-fit into the housing hole 110, for example. The mounting member 200 is mounted to the compressor housing 100 in the above manner. However, the mounting member 200 may be mounted to the compressor housing 100 by a fastening member such as a bolt. The mounting member 200 may be joined to the compressor housing 100.

A mounting hole 211 is formed in the main body portion 210. The mounting hole 211 penetrates through the main body portion 210 in the rotational axis direction. The mounting hole 211 is continuous with the partition wall hole 122 in the rotational axis direction. In the mounting hole 211, a reduced diameter portion 211 a and a parallel portion 211 b are formed. The inner diameter of the reduced diameter portion 211 a becomes smaller toward the left side (compressor impeller 9 side) in FIG. 2. The parallel portion 211 b is positioned on the left side (compressor impeller 9 side) in FIG. 2 with respect to the reduced diameter portion 211 a. The parallel portion 211 b has a substantially constant inner diameter along the rotational axis direction. The inner diameter of the parallel portion 211 b of the mounting hole 211 is approximately equal to the inner diameter of the large diameter portion 122 a of the partition wall hole 122. In this example, the case where the reduced diameter portion 211 a and the parallel portion 211 b are formed has been described. However, as long as the mounting hole 211 is formed, the shape thereof does not matter.

In the main body portion 210 of the mounting member 200, the mounting hole 211 opens at an end surface 212 on the right side (the opposite side to the compressor impeller 9) in FIG. 2. The end surface 100 a of the compressor housing 100 and the end surface 212 of the mounting member 200 are, for example, flush with each other. However, the end surface 100 a of the compressor housing 100 may be positioned on the left side (compressor impeller 9 side) in FIG. 2 with respect to the end surface 212 of the mounting member 200. That is, the mounting member 200 may protrude from the housing hole 110 to the right side (the side away from the compressor impeller 9) in FIG. 2. The end surface 212 of the mounting member 200 may be positioned on the left side (compressor impeller 9 side) in FIG. 2 with respect to the end surface 100 a of the compressor housing 100.

In the main body portion 210 of the mounting member 200, the end surface 213 on the left side (compressor impeller 9 side) in FIG. 2 is tapered. The end surface 213 is positioned leftward (compressor impeller 9 side) in FIG. 2 as it extends radially inward. The end surface 213 of the mounting member 200 and the end surface 123 of the partition wall 120 are spaced apart in the rotational axis direction. A part of the end surface 213 on the inner side in the radial direction faces the end surface 123 of the partition wall 120 in the rotational axis direction. A gap (upstream communication passage 320 which will be described later) is formed between the end surface 123 of the partition wall 120 and the end surface 213 of the mounting member 200.

Fins 220 (second partitions) are formed on the end surface 213. The multiple fins 220 are spaced apart from each other in the circumferential direction of the main body portion 210 (the rotation direction of the compressor impeller 9). In FIG. 2, a fin 220 is indicated by cross hatching that is looser than that of the rib 140 to facilitate understanding. The fins 220 are, for example, integrally molded with the mounting member 200. However, the fins 220 may be formed separately from the mounting member 200 and attached to the mounting member 200. In the sub flow passage 300, the positions of the fins 220 are fixed.

A fin 220 has an inner circumferential portion 221 and an outer circumferential portion 222. The outer circumferential portion 222 is positioned radially outward from the inner circumferential portion 221. The inner circumferential portion 221 is continuous with the outer circumferential portion 222 in the radial direction. The inner circumferential portion 221 is a portion of the fin 220 facing the end surface 123 of the partition wall 120. The inner circumferential portion 221 extends from the end surface 213 to the end surface 123 of the partition wall 120. An inner circumferential end 221 a of the inner circumferential portion 221 is substantially flush with the inner circumferential surface of the parallel portion 211 b of the mounting member 200 and the inner circumferential surface of the large diameter portion 122 a of the partition wall 120. However, the inner circumferential end 221 a of the inner circumferential portion 221 may be positioned radially outward from the inner circumferential surface of the parallel portion 211 b of the mounting member 200 and the inner circumferential surface of the large diameter portion 122 a of the partition wall 120. The outer circumferential portion 222 extends to the left side (compressor impeller 9 side) in FIG. 2 farther than the inner circumferential portion 221 does. The outer circumferential portion 222 protrudes in the gap between the outer circumferential surface of the partition wall 120 and the inner circumferential surface of the housing hole 110.

The main flow passage 10 includes the mounting hole 211, the partition wall hole 122, and the protrusion hole 132. The sub flow passage 300 is formed on the radially outer side of the main flow passage 10. The sub flow passage 300 includes the gap between the outer circumferential surface of the protrusion 130 and the outer circumferential surface of the partition wall 120 and the inner circumferential surface of the housing hole 110. The sub flow passage 300 extends annularly. The sub flow passage 300 includes the downstream communication passage 310 and the upstream communication passage 320. The downstream communication passage 310 is formed by the end surface 121 of the partition wall 120 and the end surface 131 of the protrusion 130. The upstream communication passage 320 is formed by the end surface 123 of the partition wall 120, the end surface 213 of the mounting member 200, and fins 220 (inner circumferential portions 221) adjacent to each other in the circumferential direction. Therefore, multiple upstream communication passages 320 are formed while spaced apart in the circumferential direction.

The upstream communication passages 320 communicate with the main flow passage 10. The downstream communication passage 310 communicates with the main flow passage 10 on the left side (compressor impeller 9 side, the downstream side in the flow direction of the main flow passage 10) in FIG. 2 with respect to the upstream communication passage 320.

As described above, the end surface 213 of the mounting member 200 is tapered. That is, in the upstream communication passage 320, an inner wall surface 321 on the right side (the side spaced apart from the compressor impeller 9, the end surface 100 a side) in FIG. 2 is inclined in a direction approaching the compressor impeller 9 (toward the end surface 123) as the inner wall surface 321 is closer to the main flow passage 10 (as the inner wall surface 321 extends radially inward). A cross-sectional shape of the inner wall surface 321 illustrated in FIG. 2 may be linear or curved. Since the inner wall surface 321 is inclined, the air flowing backward in the upstream communication passage 320 merges along the air flowing in the main flow passage 10. This reduces pressure loss. However, the inner wall surface 321 may extend in parallel to the radial direction. The inner wall surface 321 may be inclined in a direction away from the compressor impeller 9 (in a direction away from the end surface 123) as the inner wall surface 321 is closer to the main flow passage 10 (as the inner wall surface 321 extends radially inward).

The rib 140 is provided on the downstream communication passage 310 side in the sub flow passage 300. The downstream communication passage 310 opens between the multiple ribs 140 spaced apart from each other in the circumferential direction. A radially outer end of the downstream communication passage 310 opens between the multiple ribs 140. For example in a case where the rib 140 is spaced apart from the bottom surface 111 to the fin 220 side and the downstream communication passage 310 opens to the left side in FIG. 2 with respect to the rib 140, the air flowing backward from the downstream communication passage 310 may hit an end of the rib 140 on the side of the bottom surface 111, which may result in separation. Since the radially outer end of the downstream communication passage 310 opens between the multiple ribs 140, such separation is avoided. However, the downstream communication passage 310 may open on the left side in FIG. 2 with respect to the rib 140 as long as the effect of separation is limited to a level that does not incur a disadvantage as a result of other design conditions.

The downstream communication passage 310 faces the compressor impeller 9. A radially inner end of the downstream communication passage 310 opens on an inner circumferential surface of the compressor housing 100 facing the compressor impeller 9 in the radial direction.

The downstream communication passage 310 extends, for example, parallel to the radial direction. However, the downstream communication passage 310 may be inclined with respect to the radial direction. The downstream communication passage 310 may be inclined toward the right side (upstream communication passage 320 side) in FIG. 2 as the downstream communication passage 310 extends radially outward. The downstream communication passage 310 may be inclined toward the left side (opposite side to the upstream communication passage 320) in FIG. 2 as the downstream communication passage 310 extends radially outward.

The fin 220 is provided on the upstream communication passage 320 side in the sub flow passage 300. The outer circumferential portion 222 of the fin 220 is positioned in the sub flow passage 300. The inner circumferential portion 221 is positioned in the upstream communication passage 320.

The sub flow passage 300 is partitioned in the circumferential direction by the ribs 140 and the fins 220. That is, in the area where the ribs 140 and the fins 220 are arranged, the sub flow passage 300 is divided into multiple flow passages spaced apart in the circumferential direction.

FIG. 3 is a graph for explaining the relationship between the flow rate of the main flow passage 10 and the flow rate of the sub flow passage 300. As illustrated in FIG. 3, in an area where the flow rate of the main flow passage 10 is high, the air flows forward in the sub flow passage 300 (the air flows in the same direction as the main flow passage 10, and thus the air flows from the upstream communication passage 320 side to the downstream communication passage 310 side). As the flow rate of the main flow passage 10 is larger, the forward flow rate in the sub flow passage 300 increases.

In an area where the flow rate of the main flow passage 10 is small, the high pressure air compressed by the compressor impeller 9 flows backward in the sub flow passage 300 (the air flows in a reverse direction to the flow direction of the main flow passage 10, and thus the air flows from the downstream communication passage 310 side to the upstream communication passage 320 side). As the flow rate of the main flow passage 10 is smaller, the flow rate of the reverse flow in the sub flow passage 300 becomes larger. The air flowing backward in the sub flow passage 300 circulates to the main flow passage 10 from the upstream communication passage 320. As a result, the apparent flow rate increases, and thus the operation area on the small flow rate side expands.

The air flowing backward from the downstream communication passage 310 to the sub flow passage 300 swirls under the influence of the rotation of the compressor impeller 9. The swirling flow is in the same direction as the rotation direction of the compressor impeller 9. In the case where the sub flow passage 300 is partitioned by the ribs 140 and the fins 220, the swirling speed component of the air circulating from the upstream communication passage 320 to the main flow passage 10 is suppressed. The pressure on the intake side of the compressor impeller 9 rises, and thus the operation area on the small flow rate side further expands.

FIG. 4A is a cross-sectional view taken along line IVa-IVa in FIG. 2. FIG. 4B is a cross-sectional view taken along line IVb-IVb in FIG. 2. In FIG. 4A, the radially outer portion of the compressor housing 100 and the compressor impeller 9 are not illustrated.

In the example illustrated in FIG. 4A, three ribs 140 are formed. The ribs 140 are spaced at equal intervals in the circumferential direction of the partition wall 120. However, the ribs 140 may not be arranged at equal intervals (the intervals may vary). In the example illustrated in FIG. 4B, four fins 220 are formed. The fins 220 are spaced at equal intervals in the circumferential direction of the partition wall 120. However, the fins 220 may not be arranged at equal intervals (the intervals may vary).

The arrangement of the fins 220 in the circumferential direction relative to the arrangement of the ribs 140 in the circumferential direction is not limited to the positional relationship illustrated in FIGS. 4A and 4B. At least one of the fins 220 may face a rib 140 in the rotational axis direction. It is not necessary that one of the fins 220 faces a rib 140 in the rotational axis direction.

The thickness La of the ribs 140 in the rotation direction is less than or equal to five times the thickness Lb of the fins 220 in the rotation direction. In a case where the thickness La of the ribs 140 in the rotation direction exceeds five times the thickness Lb of the fins 220 in the rotation direction, a gap between adjacent ribs 140 becomes narrow. The flow velocity of the air flowing through the gap between the adjacent ribs 140 increases. The effect of separation at the fins 220 which will be described later increases. In this case, since the flow velocity of the air increases, separation occurs on wall surfaces of the fins 220 on the rotation direction side. Specifically, the separation flow does not adhere to the wall surfaces of the fins 220, and separation bubbles spread in the rotational axis direction. When the separation bubbles spread in the rotational axis direction, the loss increases due to the expanding scale of the separation. On the other hand, in a case where the thickness La of the ribs 140 in the rotation direction is less than or equal to five times the thickness Lb of the fins 220 in the rotation direction, the effect of separation is suppressed and the compression efficiency improves. However, the thickness La of the ribs 140 in the rotation direction may exceed five times the thickness Lb of the fins 220 in the rotation direction as long as the effect of separation is limited to a level that does not incur a disadvantage as a result of other design conditions.

The fins 220 extend in parallel to each other in the radial direction of the compressor impeller 9 (radially along the radial direction). However, the fins 220 may be inclined with respect to the radial direction of the compressor impeller 9. For example, the outer circumferential end of a fin 220 may be shifted in the rotation direction with respect to the inner circumferential end of the fin 220. The ribs 140 extend in parallel to each other in the radial direction of the compressor impeller 9 (radially along the radial direction). However, the ribs 140 may be inclined with respect to the radial direction of the compressor impeller 9. For example, the outer circumferential end of a rib 140 may be shifted in the rotation direction with respect to the inner circumferential end of the rib 140.

FIG. 5 is a table illustrating exemplary measurement results of the compression efficiency depending on the number of ribs 140 and the number of fins 220. FIG. 6 is a first graph based on the measurement results illustrated in FIG. 5. In FIGS. 5 and 6, the increase/decrease rate (%) of the compression efficiency is indicated using the case where the number of ribs 140 (the number of ribs) is three and the number of fins 220 (the number of fins) is eight as a reference value.

As illustrated in FIG. 6, in the case of three ribs 140, the compression efficiency is higher than the reference value when fins 220 are in a first range X (seven or less fins 220) In the case of six ribs 140, the compression efficiency is higher than the reference value when fins 220 are in a second range Y (two to ten fins 220). In the case of nine ribs 140, the compression efficiency is higher than the reference value when fins 220 are in a third range Z (five to thirteen fins 220).

On the basis of FIGS. 5 and 6, an appropriate range of the number of fins 220 relative to the number of ribs 140 is obtained. That is, it is assumed that N ribs 140 are arranged and M fins 220 are arranged, where N and M are natural numbers. In this example, the ribs 140 and the fins 220 are arranged such that N−4≤M≤N+4 is satisfied.

As described above, in the case where the sub flow passage 300 is partitioned by the ribs 140 and the fins 220, the swirling speed component of the air circulating from the upstream communication passage 320 to the main flow passage 10 is suppressed. The pressure on the intake side of the compressor impeller 9 rises, and thus the operation area on the small flow rate side expands. However, in a case where the number of fins 220 is too large relative to the number of ribs 140, the pressure loss increases by the effect of separation occurring at an end 223 (see FIG. 2) of a fin 220 on the rib 140 side. By arranging M fins 220 for N ribs 140 (where N−4≤M≤N+4), the pressure loss is suppressed while the swirling speed component of the air is suppressed, which improves the compression efficiency.

FIG. 7 is a second graph based on the measurement results illustrated in FIG. 5. In FIG. 7, the ranges of the number of fins 220 (ranges X′, Y′, and Z′) are set narrower than those in FIG. 6.

As illustrated in FIG. 7, in the case of three ribs 140, the compression efficiency is especially high when fins 220 are in a first range X′ (one to five fins 220). In the case of six ribs 140, the compression efficiency is especially high when fins 220 are in a second range Y′ (four to eight fins 220). In the case of nine ribs 140, the compression efficiency is especially high when fins 220 are in a third range Z′ (seven to eleven fins 220).

That is, by arranging M fins 220 for N ribs 140 (where N−2≤M≤N+2), the pressure loss is further suppressed while the swirling speed component of the air is suppressed, which further improves the compression efficiency. Here, it is possible to reduce the difference in the number of items to be arranged between the N ribs 140 and the M fins 220. In this case, the change in the flow passage area of the air (fluid) passing through the ribs 140 and the fins 220 is reduced. As a result, this can suppress the loss caused by the acceleration or deceleration of the air in the sub flow passage 300. For example, if the air is accelerated, the effect of the above-mentioned separation becomes large. It can be expected that the loss due to the effect of this separation be suppressed. Moreover, for example, when the air is decelerated, the ratio of the velocity component in the circumferential direction increases. As a result, it is predicted that a mixing loss due to merging of the main flow and the flow flowing into the main flow from the upstream communication passage 320 increases. It can be expected that this mixing loss be suppressed.

Although an embodiment of the present disclosure has been described with reference to the accompanying drawings, it is naturally understood that the present disclosure is not limited to the above embodiment. It is clear that those skilled in the art can conceive various modifications or variations within the scope described in the claims, and it is understood that they are naturally also within the technical scope of the present disclosure.

INDUSTRIAL APPLICABILITY

The present disclosure can be leveraged in a centrifugal compressor in which a sub flow passage communicating with a main flow passage is formed. 

What is claimed is:
 1. A centrifugal compressor comprising: an impeller; a main flow passage in which the impeller is arranged, the main flow passage extending in a rotational axis direction of the impeller; a sub flow passage including an upstream communication passage communicating with the main flow passage and a downstream communication passage communicating with the main flow passage on the impeller side with respect to the upstream communication passage; N first partitions fixed to the sub flow passage and arranged while spaced apart from each other in a rotation direction of the impeller; and M second partitions (where N−4≤M≤N+4) fixed on the upstream communication passage side with respect to the first partitions in the sub flow passage, the M second partitions arranged while spaced apart from each other in the rotation direction.
 2. The centrifugal compressor according to claim 1, wherein a thickness of the first partitions in the rotation direction is less than or equal to five times a thickness of the second partitions in the rotation direction.
 3. The centrifugal compressor according to claim 1, wherein the downstream communication passage opens between the multiple first partitions.
 4. The centrifugal compressor according to claim 2, wherein the downstream communication passage opens between the multiple first partitions.
 5. The centrifugal compressor according to claim 1, wherein an inner wall surface of the upstream communication passage on a side spaced apart from the impeller is inclined in a direction closer to the impeller as the inner wall surface is closer to the main flow passage.
 6. The centrifugal compressor according to claim 2, wherein an inner wall surface of the upstream communication passage on a side spaced apart from the impeller is inclined in a direction closer to the impeller as the inner wall surface is closer to the main flow passage.
 7. The centrifugal compressor according to claim 3, wherein an inner wall surface of the upstream communication passage on a side spaced apart from the impeller is inclined in a direction closer to the impeller as the inner wall surface is closer to the main flow passage.
 8. The centrifugal compressor according to claim 4, wherein an inner wall surface of the upstream communication passage on a side spaced apart from the impeller is inclined in a direction closer to the impeller as the inner wall surface is closer to the main flow passage.
 9. The centrifugal compressor according to claim 1, wherein the second partitions extend in a radial direction of the impeller.
 10. The centrifugal compressor according to claim 2, wherein the second partitions extend in a radial direction of the impeller.
 11. The centrifugal compressor according to claim 3, wherein the second partitions extend in a radial direction of the impeller.
 12. The centrifugal compressor according to claim 4, wherein the second partitions extend in a radial direction of the impeller.
 13. The centrifugal compressor according to claim 5, wherein the second partitions extend in a radial direction of the impeller.
 14. The centrifugal compressor according to claim 6, wherein the second partitions extend in a radial direction of the impeller.
 15. The centrifugal compressor according to claim 7, wherein the second partitions extend in a radial direction of the impeller.
 16. The centrifugal compressor according to claim 8, wherein the second partitions extend in a radial direction of the impeller. 